Role of myokines in the maintenance of whole body metabolic homeostasis pdf

Levels of FGF21 in skeletal muscle were increased in response to insulin stimulation and to hyperinsulinemia [32].. Interestingly, the expression of FGF21 in human skeletal muscle does n

MINERVA ENDOCRINOLOGICA

EDIZIONI MINERVA MEDICA

This provisional PDF corresponds to the article as it appeared upon acceptance.

A copyedited and fully formatted version will be made available soon.

The final version may contain major or minor changes.

Subscription: Information about subscribing to Minerva Medica journals is online at:

http://www.minervamedica.it/en/how-to-order-journals.php Reprints and permissions: For information about reprints and permissions send an email to: journals.dept@minervamedica.it - journals2.dept@minervamedica.it - journals6.dept@minervamedica.it

Role of myokines in the maintenance of whole-body

Article type: Review Article

The online version of this article is located at http://www.minervamedica.it

Role of myokines in the maintenance of whole-body metabolic homeostasis

Tatiana Y Kostrominova

Department of Anatomy and Cell Biology, Indiana University School of Medicine-Northwest, Gary, IN, USA

Corresponding author: Tatiana Y Kostrominova

Department of Anatomy and Cell Biology, Indiana University School of

Medicine-Northwest, Gary, IN, USA

E-mail: tkostrom@iun.edu

Obesity is reaching epidemic proportions in developed countries and is on the

rise in developing countries Obesity-related changes in lipid and glucose

metabolism predispose to the development of metabolic syndrome and type 2

diabetes Skeletal muscle constitutes about 40 percent of total body weight and is

unique compared to other muscle types since it is one of the most important organs

for insulin-dependent glucose metabolism in humans Abnormalities in skeletal

muscle lipid and glucose metabolism as well as abnormal accumulation of

intramyocellular lipids could predispose for the development of type 2 diabetes

Skeletal muscle synthesizes and secretes factors with

autocrine/paracrine/endocrine functions that can regulate skeletal muscle

metabolism as well as affect other organs These factors secreted by skeletal

muscle are called myokines Secretion and action of myokines is regulated by

physiological conditions Some myokines have positive effect on metabolism,

improving functions of multiple organs Yet, other myokines are released under

pathological conditions and might exacerbate abnormal metabolic functions

Expression and/or secretion of a number of myokines are regulated by exercise and

therefore might mediate positive effects of physical activity on whole-body

metabolism In the current review we summarized current knowledge on some of

the myokines with important physiological functions in lipid and glucose metabolism

A better understanding of the effects of myokines on whole-body metabolism can

aid in development of the future pharmacologic therapies for counteracting the

current worldwide obesity epidemic and obesity-mediated abnormalities

Key words: skeletal muscle, myokines, obesity, metabolism.

TEXT Introduction

Obesity has reached epidemic proportions worldwide and the rate of obesity

continues to increase According to the World Health Organization, in 2014 about

39% (1.9 billion) of adults worldwide were overweight and 13% (over 600 million)

were obese The World Obesity Federation estimates that if current trends continue,

around 3 billion adults worldwide will be overweight by the 2025 In order to combat

the increasing rate of obesity it is essential to understand molecular mechanisms

and factors that mediate the development of obesity-induced abnormalities Due to

the extensive studies performed in the last two decades the critical role of factors

secreted by adipose tissue (adipokines) in regulation of the whole-body lipid and

glucose metabolism is now well established

There are more than 600 factors secreted by adipocytes and circulating in the

blood that could have autocrine/paracrine/endocrine functions [1] Some adipokines

have beneficial effects on metabolism and improve insulin sensitivity, as well as lipid

and glucose metabolism (adiponectin, vaspin, FGF21) Their secretion usually is

decreased by obesity Other adpokines have negative effect on metabolism,

promoting insulin resistance and dyslipidemia (resistin, visfatin, RBP4) and their

secretion is increased by obesity Current medical textbooks describe adipose

tissue as an adipokine-secreting endocrine organ, with a well-established role in

whole-body metabolism Adipokines circulating in the blood can affect the function

of many organs, including skeletal muscle A number of previous and current

studies are focused on using adipokines for the development of the treatment for

obesity-induced abnormalities in humans

Currently it is much less appreciated that skeletal muscle also can secrete many

of the same factors with autocrine/paracrine/endocrine functions that are secreted

by adipocytes, as well as some muscle-specific factors These factors secreted by

skeletal muscle are called myokines This term, derived from Greek words for

“muscle” and “motion” was proposed to describe factors with endocrine function that

are produced and secreted by skeletal muscle cells [2] Skeletal muscle is the

second largest organ of the human body It is the largest target organ for

insulin-stimulated glucose utilization Therefore, abnormalities in skeletal muscle glucose

and lipid metabolism can lead to pronounced whole-body metabolic abnormalities

Skeletal muscle-secreted myokines can regulate metabolism by endocrine

mechanism acting on distant organs such as liver and adipose tissue (Figure 1)

Myokines are also capable of regulating muscle functions by an autocrine

mechanism as well as function of the located nearby tendon and bone tissues by a

paracrine mechanism (Figure 1) A number of myokines have already been

identified For many of myokines we have only limited knowledge We know that

they potentially can be myokines but their functions are not yet studied Analysis of

the secretory profile of human skeletal muscle cells has identified 305 proteins as

potential myokines [3] Expression of some of these myokines is regulated by

exercise Fifteen out of the 236 proteins first identified in cultured skeletal muscle

cells were later shown to be significantly up-regulated in human skeletal muscle in

vivo in response to strength training [4]

A number of myokines have been previously described in detail (FGF21,

adiponectin, LIF, IL-6, CTRPs, irisin, apelin, lipocalins, etc.) although the

mechanism regulating their secretion by skeletal muscle and their effect on other

organs and whole-body metabolism are not yet completely elucidated Multiple

studies suggest that myokine secretion is regulated by diverse physiological

changes including obesity, cancer cachexia, insulin resistance and exercise Unlike

adipokines, myokines are not yet mentioned in medical textbooks and are not

discussed in medical school curricula As a result, the role of myokines as regulators

of the whole-body lipid and glucose metabolism is currently underappreciated in the

medical community A better understanding of the mechanisms by which myokines

can affect obesity-induced abnormalities will help to develop future therapies

In this review we will focus on selected myokines that, in our opinion, play crucial

roles in whole-body lipid and glucose metabolism and have a potential to be used in

the future as diagnostic or therapeutic tools to diminish obesity-induced

abnormalities and prevent type 2 diabetes

Myostatin, also known as GDF-8, is a member of TGF beta family and is one of the

first described myokines [5] Binding of myostatin to the activin receptor type IIB

promotes skeletal muscle atrophy in many physiological conditions, including

denervation and fasting Myostatin also inhibits activation of skeletal muscle-specific

stem cells, satellite cells, in response to injury and slows down muscle differentiation

[6] Myostatin mediates skeletal muscle atrophy through activation of the forkhead

box O transcription factors (FoxO) leading to increased expression of

muscle-specific E3 ligases, atrogin-1 and MuRF1 (muscle RING-finger protein 1) E3 ligases

target muscle myofibrillar and intracellular proteins for degradation through the

ubiquitin–proteasome system Myostatin also increases skeletal muscle oxidative

stress via NF-κB- and NADPH-mediated signaling cascades [7] Myostatin-null mice

have significantly increased lean skeletal muscle mass when compared with wild

type littermates [5] Increased muscle mass results from both hypertrophy (increase

in muscle fiber size) and hyperplasia (proliferation of muscle stem cells to increase

the number of muscle fibers) [5] Myostatin-null mice also are protected from

diet-induced obesity, they have enhanced peripheral tissue fatty acid oxidation and

increased thermogenesis [8] Myostatin-induced catabolic processes mediate

skeletal muscle atrophy in cancer cachexia [9] Myostatin expression can be

regulated by endocrine hormones In rats hypothyroidism is associated with

increased myostatin mRNA expression [10] It is currently unclear whether

hypothyroidism in humans is also associated with increased myostatin levels in

skeletal muscle The endocrine function of myostatin could be mediated by its

effects on adipose tissue Myostatin treatment has been shown to increase

proliferation of adipocytes and inhibit their differentiation [11] Myostatin treatment

also influenced the expression and secretion of adiponectin, resistin, and visfatin by

adipocytes [11]

Brain-derived neurotrophic factor (BDNF) is a myokine that affects

myogenesis and skeletal muscle regeneration [12] The expression of BDNF is high

in myoblasts but it is decreased in differentiated myotubes [13] BDNF effects are

mediated by binding to specific cell surface receptors Human myocytes express

p75NTR and not TrkB BDNF receptors [14] BDNF is expressed at different levels in

fast and slow muscle fibers Mostly slow rat soleus muscle has 2-fold higher levels

of BDNF expression than mostly fast gastrocnemius muscle [15] BDNF has

beneficial effects on nerve growth and regeneration via paracrine mechanism [16]

Following denervation skeletal muscle increases BDNF expression and retrograde

transport of BDNF to spinal cord [16] BDNF also affects whole-body metabolism via

autocrine and endocrine mechanisms Systemic treatment of mice with BDNF

reduces total food intake and inhibits weight gain [17] These effects are mediated

by increased GLUT4 expression in skeletal muscle [17] In obese diabetic mice

BDNF treatment enhances glucose utilization in skeletal muscle and brown adipose

tissue [18]

Insulin-like growth factor 1 (IGF-1) is released by skeletal muscle and affects

skeletal muscle growth Mutant mice with reduced IGF-1 content in muscle are

~30% smaller, have reduced levels of circulating IGF-1, have smaller muscles and

decreased bone mineral density [19] Growth defects are diminished by

administration of recombinant IGF-1 [19] IGF-1 regulates whole-body metabolism

Skeletal muscle-specific inactivation of IFG-1 receptors leads to the development of

insulin resistance [20] High fat diet-induced obesity in mice reduces expression of

IGF-1 in intact muscles and diminishes up-regulation of IGF-1 in response to muscle

injury [21]

Fibroblast Growth Factor 2 (FGF-2), also known as basic FGF, is a myokine that

has pleiotropic effects in a range of cell types During myogenic differentiation

expression of FGFs and FGF receptors are downregulated [22] FGF-2 secreted by

muscle cells may act as a paracrine and autocrine regulator of skeletal muscle

development in vivo [23] During ischemia, increased expression of FGF-2 in

skeletal muscle promotes angiogenesis via its paracrine effects on blood vessels

[24] Treatment with FGF-2 stimulates proliferation of tendon cells [25] suggesting

that skeletal muscle released FGF-2 may have paracrine effects on tendon FGF-2

also plays an important role in bone formation and remodeling Mechanical

wounding of muscle cells increases release of FGF-2 into media [26] and may have

paracrine effect on tendon and bone

Fibroblast growth factor 21 (FGF21) is a well described adipocytokine/

hepatokine/ myokine with glucose and lipid-lowering properties The liver is

considered the major source of the plasma FGF21 Under normal physiological

conditions the level of hepatic FGF21 expression is low but it increases during

prolonged fasting, liver injury, exposure to toxic chemicals and viral infections [27]

In mice chronic infusion of FGF21 improves insulin responsiveness due to the

reduced diacylglycerol content and reduced protein kinase C activation in skeletal

muscle [28] In human clinical trials FGF21 treatment of obese type 2 diabetic

patients improved total body weight, dyslipidemia, insulin sensitivity and adiponectin

levels [29] [27] FGF21 works through the FGF receptors (FGFR1 and FGFR2) and

requires transmembrane protein beta-Klotho (KLB) as a co-receptor for activation of

the intracellular signaling pathways [27] Recent studies have focused on the

development of novel FGF21 mimetics with improved stability and pharmacokinetics

that could be applied for improving lipid and glucose metabolism [30]

Izumiya and colleagues reported that FGF21 is a myokine regulated through

Akt-dependent mechanisms [31] Levels of FGF21 in skeletal muscle were increased in

response to insulin stimulation and to hyperinsulinemia [32] Interestingly, the

expression of FGF21 in human skeletal muscle does not correlate with FGF21

levels in the plasma [32] mRNA expression of FGF21 in skeletal muscle is

significantly lower than in liver but similar to the expression in subcutaneous fat [33]

These might explain the lack of correlation between plasma and skeletal muscle

FGF21 levels Nevertheless, FGF21 at low levels could have considerable local

autocrine effects on skeletal muscle metabolism This hypothesis is supported by

studies showing direct effects of FGF21 on increased glucose uptake in cultured

human skeletal muscle mediated by increased glucose transporter 1 (GLUT1)

expression [34]

Skeletal muscle-specific overexpression of uncoupling protein 1 (UCP1)

increases expression of mRNA for FGF21 (several–hundred-fold) as well as its

co-receptor KLB (~2-fold) but not FGFR1 [33] Similarly, in a mouse model of diabesity

on a NZO background, skeletal muscle-specific UCP1 overexpression diminished

obesity and insulin resistance via induced expression of FGF21 [35] These suggest

that under certain physiological conditions when expression of uncoupling proteins

in skeletal muscle is increased [33] or the cellular stress response activating

transcription factor (ATF) 4 cascade is activated [33], the expression of FGF21 in

skeletal muscle and in plasma could be significantly up-regulated Mitochondrial

stress and dysfunction is characterized by increased production of reactive oxygen

species, activation of p38 MAPK and ATF2, leading to the increased expression of

FGF21 in skeletal muscle [36] FGF21 expression is controlled by myogenic factor

MyoD [36] Constitutive activation of the mammalian target of rapamycin complex 1

(mTORC1) in mice due to the muscle-specific depletion of tuberous sclerosis

complex 1 (TSC1) triggers endoplasmic reticulum stress and activation of protein

kinase RNA-like ER kinase, eukaryotic translation initiation factor 2! and ATF4 [37]

As a consequence skeletal muscle increases synthesis and release of FGF21

leading to increased whole-body insulin sensitivity and lipid oxidation [37] Similarly,

in skeletal muscle, enhanced activity of eukaryotic translation initiation factor

4E-binding protein 1 (4E-BP1), a key downstream effector of mTORC1, increases

FGF21 synthesis [38]

High levels of plasma FGF21 correlating with increased FGF21 expression in

skeletal muscle were detected in patients with mitochondrial dysfunctions in skeletal

muscles due to a deficiency of iron-sulfur clusters [39] Treatment of cultured human

skeletal muscle cells with FGF21 in a time-dependent manner increased mRNA

expression of KLB and at the same time decreased palmitate-induced insulin

resistance by inhibiting activation of Akt and NF-κB [40] This phenomenon could

represent a protective mechanism activated in response to metabolic and oxidative

stress

Insulin-sensitizing effects of FGF21 were reported to be mediated by adiponectin

[41] Skeletal muscle–specific delivery of adenoviral FGF21 vector increased

plasma FGF21 as well as adiponectin levels [42] These correlate with the

decreased negative effects of myocardial infarction in mice [42] suggesting

protective effects of FGF21 and adiponectin against infarction-induced stress

There might be significant differences in the whole-body response to FGF21

based on the tissue source of the secreted FGF21 as well as duration of the FGF21

exposure A recent report by Berti and colleagues [43] showed that unhealthy

(insulin-insensitive) obese subjects had two-fold higher levels of plasma FGF21 than

body fat-matched healthy obese subjects Fatty liver might be the predominant

source of plasma FGF21 in unhealthy obese subjects [43] Prolonged treatment of

cultured human pre-adipocytes with FGF21 reduced adiponectin expression and

release and increased release of leptin and interleukin-6 [43] These studies

suggest that short-term treatment with FGF21 (once a day injections) could have

beneficial effects on whole-body lipid and glucose metabolism To the contrary,

chronically present in the plasma high levels of FGF21 could potentially promote

hyperlipidemia and insulin resistance

Adiponectin is well known adipocytokine that belongs to the complement

component 1q (C1q) family In the plasma, adiponectin can be found in high-,

middle- and low-molecular weight multimeric forms Reduced levels of

high-molecular-weight adiponectin are highly associated with increased risk for

cardiovascular abnormalities [44] Adiponectin has insulin sensitizing,

anti-inflammatory, anti-atherogenic and anti-apoptotic effects and therefore could

alleviate some of the negative effects of obesity Adiponectin mRNA and protein

expression are increased during skeletal muscle differentiation [45] Skeletal muscle

can increase adiponectin synthesis in response to physiological stress For

example, increased inflammation and cytokine exposure in response to injection of

lipopolysaccharides causes increased adiponectin mRNA and protein expression in

mice [46] Adiponectin works through two receptors AdipoR1 and AdipoR2 both of

which are expressed in skeletal muscle [47] AdipoR1 expression is significantly

higher in muscle while and AdipoR2 is highly expressed in liver [47] In muscle,

adiponectin promotes the interaction of AdipoR with APPL1 (adaptor protein

containing pleckstrin homology domain, phosphotyrosine binding domain and

leucine zipper motif) potentiating adiponectin signaling and promoting

adiponectin-induced GLUT4 membrane translocation, glucose uptake and lipid oxidation [48]

Expression of AdipoR1, AdipoR2 and APPL1 is increased during skeletal muscle

differentiation [45] Skeletal muscle unloading causes muscle atrophy via increased

proteolysis and is associated with decreased expression of AdipoR1 but not

AdipoR2 [45] To the contrary, skeletal muscle functional overloading is associated

with increased expression of adiponectin and APPL1 [45]

The link between obesity and alterations in adiponectin expression is well

described Obesity, increased visceral fat accumulation, type 2 diabetes and

cardiovascular disease are highly correlated with down-regulated adiponectin

secretion by adipocytes, decreased adiponectin signaling in liver and skeletal

muscle and lower plasma adiponectin levels [49] Polymorphisms at the adiponectin

locus may be associated with low levels of circulating adiponectin levels, insulin

resistance, and atherosclerosis [50] A high fat diet decreases skeletal muscle

AdipoR1 expression predisposing to the potential metabolic abnormalities [51]

A protective effect of adiponectin in muscle under diverse physiological

conditions is well documented In cultured skeletal muscle cells adiponectin

increases Akt phosphorylation/activation and promotes glucose uptake [52]

Adiponectin can also promote mitochondrial biogenesis in skeletal muscle via

PGC-1 alpha signaling pathway and suppression of MAPK phosphatase-PGC-1 [53]

Adiponectin inhibits (NF)-κB-inducing kinase (NIK), an upstream kinase of NF-κB

pathway, reducing inflammation and promoting insulin sensitivity in skeletal muscle

[54] Adiponectin exerts insulin-sensitizing effects in liver and skeletal muscle via

adenosine monophosphate-activated protein kinase and proliferator-activated

receptor alpha activation Increased levels of adiponectin could also have a

beneficial effect during skeletal muscle injury and oxidative stress A mouse model

of Duchenne muscular dystrophy (mdx mice) have muscle degeneration and

increased muscle inflammation, accompanied by the reduced levels of adiponectin

in the plasma [55] When mdx mice were crossed with mice overexpressing

adiponectin muscles were protected from injury and displayed higher force and

endurance properties [55] Obesity is associated with increased oxidative stress and

accumulation of damaged proteins and organelles Despite decreased levels of

adiponectin in plasma, skeletal muscle of obese mice has increased levels of

adiponectin expression [56] It was previously reported that beneficial metabolic

effects of adiponectin in skeletal muscle of high fat diet-treated mice are mediated

by autophagy [57], probably via removal of damaged/ nonfunctional organelles In

mice, activation of adiponectin receptors by the small molecule agonist AdipoRon

improves insulin resistance and glucose uptake [58] and potentially could be

developed into a clinically-approved drug for treatment of obesity-induced

abnormalities

CTRP proteins Recently fifteen novel secreted proteins of the C1q family were

identified (C1q/TNF-related proteins; CTRP1–15) that share structural and

functional similarity with adiponectin [59] CTRP proteins have in common four

distinct domains: a signal peptide at the N terminus, a variable region, a collagenous

domain (Gly-X-Y repeats), and a C-terminal globular domain C1q The majority of

CTRPs are considered to be adipokines due to their preferential expression in

adipose tissue, but CTRP15/myonectin is a skeletal muscle-specific myokine that

regulates lipid metabolism in liver and adipose tissue [60] Some other CTRPs are

also expressed in skeletal muscle (reviewed in [59] [61]) but their role as myokines

is not yet well studied For example, it is reported that CTRP1, CTRP3, CTRP5,

CTRP12, CTRP15 are expressed in skeletal muscle but very little is known about

their contribution to the regulation of the whole-body lipid and glucose metabolism

CTRP1 is a 35-kDa glycoprotein that functions as adipokine/myokine/hepatokine

and is most highly expressed in adipocytes CTRP1 is highly up-regulated in

response to lipopolysaccharide treatment [62] Expression of CTRP1is regulated by

TNF-alpha and interleukin-I beta [62] CTRP1 has higher expression levels in

adipose tissue of genetically modified obese rats and mice when compared with

lean controls [62] In the obese state, synthesis and secretion of adiponectin in

adipose tissue is highly down-regulated Therefore a high expression of CTRP1 in

the obese state might represent a compensatory effect In skeletal muscle cells

CTRP1 promotes fatty acid oxidation and inhibits acetyl-CoA carboxylase [63]

When CTRP1 overexpressing transgenic mice are placed on a fat or a

high-sucrose diet, they have increased glucose uptake and diminished insulin resistance

[63]

CTRP3, cartonectin, is an adipokine/myokine/hepatokine involved in multiple

physiological processes It was initially identified as a TGF beta-responsive gene in

growing cartilage [64] Expression of CTRP3 in muscle is regulated via the

transforming growth factor beta pathway and is increased during myogenic

differentiation of muscle cells in vitro [65] CTRP3 can stimulate proliferation and

differentiation of chondrogenic precursor cells via an ERK signaling pathway,

playing an important role in cartilage development [66] CTRP3 also promotes

angiogenesis by stimulation of proliferation and migration of endothelial cells [67]

Therefore, CTRP3 can potentially be involved in the regulation of blood vessels and

bone development by growing skeletal muscle during embryonic development and

physiological adaptations Levels of CTRP3 in plasma are increased with fasting

and decreased in diet-induced obesity [68] Treatment of mice with CTRP3 can

decrease plasma glucose levels but does not affect levels of insulin and adiponectin

[68] CTRP3 also educes lipopolysaccharide-induced release of IL-5 and TNF in

monocytes from healthy controls but not from type 2 diabetic patients [69]

CTRP5 has significant effects on glucose metabolism and mitochondrial content in

skeletal muscle CTRP5 expression is increased in insulin-resistant rodents and in

mitochondrial DNA-depleted muscle cells [70] Treatment of myocytes with

recombinant CTRP5 induces AMPK phosphorylation and increases glucose uptake

through the stimulation of GLUT4 translocation to the plasma membrane [70] This

is very similar to the effects produced by adiponectin CTRP5 does not induce

phosphorylation of IRS-1 and Akt, suggesting that it works independently from the

insulin signaling pathway [70] In response to aerobic exercise in humans, serum

CTRP5 levels are negatively correlated with maximal oxygen consumption,

mitochondrial density and adiponectin levels [71]

CTRP12, also called adipolin, is an adipokine/myokine that improves insulin

sensitivity and glycemic control in mouse models of obesity and diabetes [72]

CTRP12 is synthesized and secreted as full-length (fCTRP12) and cleaved

(gCTRP12) isoforms [72] CTRP12 expression is decreased in leptin-deficient

ob/ob, and diet-induced obese mice The expression of CTRP 12 in adipocytes was

restored in response to treatment with anti-diabetic drug rosiglitazone [72] CTRP12

improves insulin sensitivity by enhancing insulin signaling in liver and adipose tissue

via a PI3K-Akt signaling pathway [72] In adipocytes, obesity and inflammation

decreased expression of CTRP12 via JNK-dependent down-regulation of

transcription regulator Kruppel-like factor 15 (KLF15) [73] Currently it is not known

whether the expression and/or secretion of CTRP12 in skeletal muscle are

regulated via PI3K-Akt or JNK/KLF15 signaling pathways

CTRP15, also known as myonectin, is a myokine that promotes fatty acid uptake,

controls cellular autophagy and links skeletal muscle to the whole-body lipid

homeostasis, mostly through effects on lipid and glucose metabolism in liver and

adipose tissue [60] Although CTRP15 is expressed by several tissues, the highest

mRNA expression levels are found in skeletal muscle [60] Therefore CTRP15 is

considered primarily to be a myokine, predominantly expressed by skeletal muscle

and released into serum The deduced mouse myonectin protein consists of five

domains: a signal peptide for secretion, an N-terminal domain-1, a short collagen

domain with six Gly-X-Y repeats, an N-terminal domain-2, and a C-terminal

C1q/TNF-like domain [60] Secreted CTRP15 can form disulfide-linked oligomers

and is able to form heteromeric complexes with other CTRP molecules [60] When

co-expressed in mammalian cells, CTRP15 forms heteromeric complexes with

CTRP2 and CTRP12, and, to a lesser extent, with CTRP5 and CTRP10 [60] mRNA

expression of CTRP15 is increased during myogenic differentiation from

undifferentiated myoblasts to differentiated myotubes [60] This suggests that

concentrations of CTRP15 in the serum are regulated during embryonic/fetal

development and correlate with the degree of differentiation of skeletal muscle The

expression is up-regulated by increased calcium and cAMP concentrations but it is

not changed in response to insulin and AMP-activated protein kinase activator

(AICAR) treatment [60] In response to exercise, levels of CTRP15 mRNA

expression in skeletal muscle are increased Interestingly, the expression is more

highly up-regulated in mostly fast plantaris muscle when compared with mostly slow

soleus muscle [60] During fasting, the CTRP15 level in skeletal muscle is low but it

is increased in response to glucose and lipids both in vivo and in vitro [60]

Up-regulation of mRNA expression in response to re-feeding was ~twenty-fold higher in

slow soleus muscle when compared with fast plantaris muscle [60] These data

suggest differential regulation of CTRP15 expression in response to exercise and

re-feeding between fast and slow muscles Therefore changes in skeletal muscle

composition under physiological conditions, such as a relative decrease of slow

muscle fibers with prolonged obesity or a relative increase of slow fibers with aging,

can affect CTRP15 expression Females appeared to have higher levels of CTRP15

in the serum during fasting and re-feeding, although this trend did not reach

statistically significant levels [60] High fat diet significantly reduces CTRP15 mRNA

expression in skeletal muscle and levels of CTRP15 protein in the serum [60]

Treatment of cultured adipocytes and hepatocytes with CTRP15 increases palmitate

intake to the same degree as treatment with insulin [60] In cultured adipocytes and

hepatocytes, recombinant CTRP15 enhances fatty acid uptake through

transcriptional upregulation of genes (e.g., CD36, FATP1, Fabp1 and Fabp4) known

to be involved in fatty acid uptake [60]

Ghrelin is a hormone that has a variety of functions in multiple organs Ghrelin is

expressed in skeletal muscle [74] and it promotes differentiation of skeletal muscle

cells via p38 kinase signaling [75].Ghrelin inhibits skeletal muscle apoptosis and

enhances cellular signaling for autophagy, resulting in overall myoprotective effects

on doxorubicin-induced toxicity in skeletal muscle [76] Due to their myoprotective

effects ghrelin [77] and ghrelin receptor agonists [78] emerge as a promising

treatment option for cancer cachexia Ghrelin has positive effects on glucose

utilization Treatment with unacylated ghrelin improves insulin signaling and glucose

uptake in skeletal muscle of diabetic mice [79] Similarly, acyl-ghrelin treatment of

rats on a high fat diet decreases muscle inflammation and lipid accumulation [80]

Ghrelin treatment of rats induces lipogenic and glucogenic patterns of gene

expression and increases triglyceride content in the liver [81] At the same time in

skeletal muscle mitochondrial oxidative enzyme activities are increased and

triglyceride content is reduced [81] This suggests that ghrelin treatment has

opposite effects on lipid metabolism in liver and skeletal muscle In various organs

ghrelin expression is also regulated differently by physiological stimuli For example,

in rats twelve weeks of exercise increased acyl-ghrelin levels in skeletal muscle and

in plasma while content of ghrelin in the fundus of stomach was decreased [74]

Musclin is a myokine that is induced during differentiation of skeletal muscle cells

[82] Insulin increases levels of musclin expression, while compounds known to

increase cellular content of cyclic AMP epinephrine, isoproterenol, and forskolin

decrease musclin expression levels [82] Recombinant musclin significantly

reduces insulin-stimulated glucose uptake and glycogen synthesis in myocytes [82]

The levels of musclin expression are low in the fasting state but increase upon

re-feeding [82] Musclin expression can be regulated by transcription factor FoxO1

which is a key regulator of muscle atrophy Musclin mRNA level is markedly

downregulated in gastrocnemius muscle of Foxo1 transgenic mice Musclin levels

are also much lower in skeletal muscle of streptozotocin-treated insulin-deficient

mice when compared with muscles from control mice [82] Musclin protein contains

a region homologous to the natriuretic peptide family, and a putative serine protease

cleavage site, similar to the natriuretic peptide family [82] Moreover, musclin

specifically binds to the natriuretic peptide receptor 3 and competes for receptor

binding with atrial natriuretic peptide [83] These data suggest that musclin can

modulate effects of atrial natriuretic peptide

Irisin is a recently described secreted myokine that could have potential for

normalization of lipid and glucose metabolism [84] Irisin is a skeletal

muscle-secreted protein representing cleaved by proteolysis extracellular part of membrane

fibronectin type III domain–containing protein 5 (FNDC5) Although irisin is

expressed in adipose tissue, the levels of expression of irisin in human muscle are

200-fold higher than in adipocytes [85] Expression of irisin is regulated via a

PGC-1alpha pathway [84] Irisin transforms white adipocytes into brown adipocytes by

increasing expression of uncoupling proteins via p38 and Erk MAP kinase signaling

pathways [86] Browning of adipocytes promotes thermogenesis and energy

expenditure, having beneficial effects on whole-body lipid metabolism In skeletal

muscle cells irisin increases oxidative metabolism via a PGC-1 alpha pathway and

up-regulates irisin mRNA expression by autocrine mechanism [87] Patients with

type 2 diabetes have decreased serum levels of irisin [88] Levels of serum irisin

negatively correlate with body mass index, percent of fat mass and waist to hip ratio,

and positively correlate with insulin sensitivity [85] Moreover, irisin can have

beneficial effects in cancer prevention It has been reported that irisin promotes

apoptosis of malignant breast epithelial cells and enhances the cytotoxic effects of

doxorubicin in these cells [89]

Visfatin, is an adipokine/myokine implicated in glucose metabolism, cell

differentiation and tissue regeneration In skeletal muscle visfatin expression is

regulated by TNF, IL-6, and glucocorticoids Treatment with recombinant visfatin

modulates the expression of myogenic transcription factors required for

differentiation [90] Treatment of muscle cells with visfatin also increases glucose

uptake via increased GLUT4 expression and translocation to the plasma membrane

[91] The effects on glucose uptake are mediated by the AMPK and p38 MAPK

pathways [91] In another study, visfatin treatment increased glucose transport in

muscle but had no effect on fatty acid oxidation [92]

Apelin is an adipokine/myokine that is involved in the regulation of glucose

metabolism, lipolysis, blood pressure, cardiovascular and fluid homoeostasis, food

intake, cell proliferation, and angiogenesis In cultured muscle cells apelin treatment

increases glucose uptake and Akt phosphorylation [93] Apelin-null mice have

diminished insulin sensitivity, hyperinsulinemia and decreased adiponectin levels in

the blood Soleus muscles of apelin knockout mice have decreased insulin-induced

Akt phosphorylation Apelin and apelin receptor expression levels are regulated by

obesity Administration of apelin to apelin-null and db/db mice results in improved

insulin sensitivity [93] In mice, treatment with apelin increases glucose utilization in

skeletal muscle and adipose tissue [94] This effect is regulated via endothelial NO

synthase, AMP-activated protein kinase, and Akt in the soleus muscle of

apelin-treated mice [94] Apelin treatment also increases UCP3 expression levels in

skeletal muscle and serum adiponectin levels in plasma [95] In rats on a high fat

diet apelin level in plasma, and apelin and apelin receptor expression is increased in

gastrocnemius (mostly fast) muscle and in adipose tissue [96] Interestingly, in

mostly slow soleus muscle, apelin expression was not changed This suggests

differences in regulation of apelin expression between slow and fast skeletal muscle

fibers Apelin has recently been described as a novel exercise-regulated myokine

with positive effects on insulin sensitivity [97], [98] Treadmill running ameliorates

high fat diet-induced insulin resistance, down-regulates apelin and apelin receptor

expression in adipocytes and apelin levels in plasma [96] At the same time,

running increases levels of expression of apelin and apelin receptors in muscles

[96] This suggests differences in regulation of apelin expression in different tissue

types Patients with type 2 diabetes have significantly higher serum apelin levels,

compared to non-diabetic control subjects [99] In non-diabetic male subjects, eight

weeks of endurance training had produced two-fold increase in apelin mRNA levels

in vastus lateralis muscle but not in adipose tissue [97], indicating differences in

exercise-induced regulation of apelin expression in different tissue types

Lipocalins (LCNs) have recently emerged as a myokine family involved in the

regulation of insulin sensitivity [100] LCNs can have both positive as well as

negative effects on the regulation of insulin sensitivity [100] The recently described

circulating LCN13 is markedly reduced in mice with genetic or diet-induced type 2

diabetes The restoration of LCN13 improves insulin resistance, hyperglycemia, and

glucose intolerance LCN13 can directly enhance insulin actions in cultured

adipocytes and therefore acts as an endogenous insulin sensitizer to regulate

glucose metabolism [101] Both LCN13 mRNA and protein are highly expressed by

skeletal muscle [101] Factors regulating LCN13 expression in skeletal muscle are

currently unknown

Interleukins (IL) are pro- and/or anti-inflammatory molecules highly expressed by

immune cells Some of the interleukins are secreted by adipocytes and by muscle

cells in response to diverse physiological stimuli and therefore function as

adipokines and/or myokines, respectively Obesity and type 2 diabetes are

associated with low grade inflammation and dysregulated secretion of interleukins

Interleukins reported to function as myokines include IL-4, IL-6, IL-7, IL-8, IL-15,

leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), cardiotrophin-1

(CT-1), and oncostatin M

IL-6 is an adipokine/myokine with diverse physiological functions Depending on the

physiological conditions and the source of IL-6 secretion, it can have either positive

or negative effects on metabolism and muscle catabolic/anabolic processes

(reviewed in [102]) IL-6 signaling is mediated by the IL-6 receptor alpha and

membrane glycoprotein 130 complex Treatment of muscle cells with IL-6 promotes

myogenic differentiation and increases mRNA expression of GLUT4, MEF2, PPAR,

UCP2, and FATP4 [103] IL-6 treatment also increases mRNA expression of IL-6

itself suggesting autoregulation of expression [103] IL-6 increases glucose uptake

and fatty acid oxidation in skeletal muscle [103] [104] Glucose uptake stimulated by

IL-6 requires the PI3-kinase signaling pathway, while fatty acid oxidation is mediated

by AMPK signaling [103] Muscle-specific IL-6 deletion caused a sex-dependent